Clarify arbiter's role within a replica set with copy-editing.

replica-set-arbiter
New flesch-ease: 26.074225434492007 vs Old: 20.134938398357292
New flesch-level: 16.88124832887701 vs Old: 18.794657650640804

replica-set-members
New flesch-ease: 22.845720269272135 vs Old: 13.691667528772797
New flesch-level:16.495703565672844 vs Old: 19.769305142463043

replica-set-primary
New flesch-ease: 28.57250000000002 vs Old: 22.513859512251514
New flesch-level:15.66218253968254 vs Old: 17.58228328710125

replica-set-secondary.json
New flesch-ease: 16.458161392297512 vs Old: 5.042060427574199
New flesch-level:18.568842584724162 vs Old: 22.441420157068062